1. Field of the Invention
The present invention relates to an electronic circuit for tuning an active filter, more specifically to a circuit having an array of switched linear electronic elements such as capacitors, which defines a time constant (e.g., an RC time constant), wherein the array is switched according to a digital code supplied to the array to adjust the total capacitance of the array to approximately a desired design value to maintain the time constant within a defined range.
2. Background of the Invention
In the field of electronics active filter elements are widely used and, hence, such filter elements are increasingly produced as mass products in the form of integrated circuits. Active filters including resistors and capacitors are particularly subject to an integration process due to the possibility of easily incorporating passive linear elements, such as resistors and capacitors, into a manufacturing process for integrated circuits. In low to medium frequency applications (few Hz to several hundred kHz) the amplifiers used in active RC filters can be considered xe2x80x9cidealxe2x80x9d amplifiers, i.e., they have infinite gain and zero input current, so that the characteristics of the filter are substantially determined by the RC feedback network of the amplifier. Unavoidable variations of parameters in the manufacturing process and variations during operation of integrated active filters, however, result in deviations of the actual value of individual elements compared to their design value up to xc2x130%. It is therefore not unusual for integrated circuits to exhibit RC time constants that differ by 50% from their design value. Accordingly, the frequency response of such filters differs in the same magnitude and, thus, clearly restricts the possible applications of these filters, since the designer has to take account of the wide range of variations.
One conventional solution to this problem has been to use integrated active filter circuits in combination with external high precision resistors and capacitors to compensate for the above-mentioned variations. This solution, however, partially offsets the advantages offered by integrated circuits, such as low cost and small size of the filter circuit.
Accordingly, fully integrated active filters have been provided that have a tunable RC time constant to compensate for variations in the manufacturing process and the operating conditions of the filter, such as temperature and supply voltage variations. One way to achieve a tunable RC time constant is to provide xe2x80x9cactivexe2x80x9d resistors, i.e., resistors fabricated as MOSFETs instead of passive resistor elements, and control the MOSFETs to provide a required resistance. In such an arrangement, a feedback circuit measures the actual RC time constant of the filter with reference to, for example, a clock frequency, and provides a corresponding signal to the MOSFETs to continuously adjust their resistance to attain the required time constant. This solution, however, necessitates a continuous input signal for the MOSFETs and thus increases power consumption of the filter circuit. Moreover, this approach is disadvantageous when a low supply voltage is used, for example, as low as about 1 V, since the MOSFETs typically require a threshold voltage of about 1 V to become conducting and, hence, the MOSFETs cannot provide a sufficient variable control range to compensate for the large variations of the active filter.
An alternative approach is to produce tunable filters comprising linear resistors instead of MOSFET resistors, and a tunable array of capacitors. This is proposed by A. M. Durham, J. B. Hughes and W. Redman-White in xe2x80x9cCircuit Architectures for High Linearity Monolithic Continuous Time Filtersxe2x80x9d, IEEE Transactions on Circuits and Systemsxe2x80x94II: Analog and Digital Signal Processing, Vol. 39, No. 9, September 1992, pp. 651xe2x80x9457. According to this technique the RC time constant of the filter is measured and compared with the nominal design value. The number of active capacitors in the array, that is, the number of capacitors actually connected to the RC network of the filter amplifier, is adjusted to keep the filter circuit within a desired RC range. Due to the employment of passive resistors instead of MOSFET elements, the filter is highly linear. Moreover, the RC time constant of the filter is determined by a digital code supplied to the array that may conveniently be stored in a latch once the digital code has been established. Although the accuracy of the RC time constant is limited by the available number of switchable capacitors in the array, and, hence, by the number of bits of the digital code, a range of xc2x15% to xc2x110% for the corner frequency of the filter is sufficient for many low to medium frequency applications so that the filters can be tuned with a relatively low number of capacitors in the array.
Typically, the RC time constant of the filter is determined by generating a pulse signal having a pulse length that represents the time constant of the filter to be tuned. Thereafter the pulse signal is compared to the nominal design value and converted into a digital code to adjust the RC time constant approximately to the design value. These tuning circuits, however, are often complex and power consuming and therefore leave place for improvements.
In view of the above problems, a need exists for an improved and efficient circuit for tuning an active filter.
According to one embodiment, a circuit includes an active filter having a plurality of first linear electronic elements arranged as a switchable array and at least one second linear electronic element, the first linear electronic elements having an electrical characteristic represented by a first design value and the second linear electronic element having an electrical characteristic represented by a second design value, the first and second linear electronic elements substantially determining a time constant of the active filter. The circuit further includes a tuning circuit for outputting a digital code n to the array of first linear electronic elements to adjust the time constant of the active filter to approximately a predefined value. The tuning circuit comprises a first linear tuning element having an electrical characteristic represented by the first design value times a first predefined factor Kc, and a second linear tuning element having an electrical characteristic represented by the second design value times a second predefined factor KR. Moreover, the tuning circuit comprises a signal generator configured to generate a signal indicative of a time constant determined by the first and second linear tuning elements, a backward counter having a reset input for setting the backward counter to an initial value xmax in response to a reset signal, a clock input connected to receive a clock signal, an enable input connected to the signal generator to receive a pulse signal for starting the counter with the initial value xmax and stopping the counter at the end of the pulse length to generate a count value xn, and a count output for providing the count value xn. Additionally, the tuning circuit comprises a decoder for converting the count value xn into a digital code n for switching the array of first linear electronic elements.
According to a further embodiment a circuit includes an active filter having a plurality of first linear electronic elements at least some of which are arranged as a switchable array and a second linear electronic element, wherein each of the first linear electronic elements corresponds to a first design value and the second linear electronic element corresponds to a second design value, whereby the first and second linear electronic elements substantially determining a time constant of the active filter. The circuit further comprises a first linear tuning element having an electrical characteristic determined by the first design value and a second linear tuning element having an electrical characteristic determined by the second design value, wherein the first and second tuning element define a tuning time constant. Additionally, the circuit includes means for generating a signal indicative of the tuning time constant, means for providing an output count value on the basis of an initial count value representing a relationship between the time constant and the tuning time constant; and means for providing a digital code to the switchable array on the basis of the output count value in order to adjust the time constant.
In a further embodiment a method of tuning an active filter is provided. The active filter comprises a plurality of first linear electronic elements at least some of which are arranged as a switchable array, and a second linear electronic element, each of the first linear electronic elements corresponding to a first design value and the second linear electronic element corresponding to a second design value, wherein the first and second linear electronic elements substantially determining a time constant of the active filter. The method comprises generating a signal indicative of a tuning time constant defined by a first linear tuning element and a second linear tuning element, wherein an electrical characteristic of the first linear tuning element is determined by the first design value, and an electrical characteristic of the second linear tuning element is determined by the second design value. Moreover, the method includes starting and stopping, on the basis of said signal, a counter operated at a clock frequency to generate a count value, wherein an initial count value of the counter is determined by the time constant and the tuning time constant.